The present invention relates to a degreasing method for metal materials such as iron, zinc, aluminum and the like, and alloyed materials of these. In detail, the invention relates to a method for degreasing a metal surface etc. by using a degreasing solution which shows a very low foaming character and excellent degreasing capability which does not contain a phosphorous compound that may adversely affect environment.
Hitherto, in the field of metal surface processing, especially in that of metal processing industries, in advance of the conversion coating process by zinc phosphate and the like , degreasing treatment is applied for a matter to be processed to remove oil and fat (for example, a mineral oil, animal and vegetable oils etc.) attached to the matter surface.
Since the degreasing process is not a final one and is carried out prior to processing, such as a conversion coating process, an ingredient in the degreasing solution such as remaining by itself or as its reaction product on the surface of a matter to be processed may adversely affecting the conversion coating process.
In general, a degreasing solution has been adopted such as containing a builder having an acid or alkali as a main body and also, containing a nonionic or an anionic type surfactant as a main ingredient.
The phosphates such as an alkali metal phosphate or alkali metal condensed phosphate or the like have been used as an useful builder to assist the degreasing capability. Recently, it has been pointed out that the phosphates cause eutrophication and pollution in lakes and marshes. Accordingly, in the degreasing of metal surfaces, there has been desired the development of a degreasing solution, free of any phosphates.
On the other hand, increasingly there are instances where a matter is to be processed, like an automobile body, consisting of two kinds or more of metal materials such as a composite structure material, which contains not only an iron material and a zinc alloy material, but also an aluminum alloy material. Where a matter to be degreased consists of two or more kinds of metal materials, the formulation of the degreasing solution and processing conditions must be chosen so as to be processed excellently in a conversion coating process.
An example of a degreasing solution without any combined phosphate is disclosed in Japanese Examined Patent Publication, showa 61-25784. This degreasing solution is an alkaline degreasing composition consisting of a nonionic surfactant, builder, hydrosulfite and succharide. If a calcium or magnesium salt is added to this degreasing solution, deterioration of the degreasing capability occurs. Also, if the aforementioned composite structure material is degreased by this degreasing solution, it is not processed excellently in conversion coating process.
A method of degreasing with which the aforementioned composite structure material can have an excellent conversion coating performance has been proposed in Japanese Official Patent Provisional Publication, showa 62-199787. In this method, when a matter to be processed is processed by an alkali silicate-based degreasing agent, a degreasing solution is controlled so as to have its pH at 10.5 or more and to keep the combined Si concentration satisfactorily for the following equation (a). However, the degreasing solution used in this method has a phosphate as an alkali builder. When this degreasing solution does not contain the phosphate, it does not display sufficient degreasing capability.
Meanwhile, in a case where a spraying method is adopted in a processing facilities for decreasing metal surfaces, the degreasing solution must be of an especially low foaming character. Conventional degreasing solutions comprise low foaming surfactants such as disclosed in Japanese Examined Patent Publication, showa 36-4373 and Japanese Official Patent provisional Publication, showa 54-86508. In Japanese Examined Patent Publication, showa 36-4373, surfactants denoted by the following general formula (1) and in Japanese Official Patent Provisional Publication, showa 54-86508, surfactants denoted by the following general formula (2) are, respectively, used. ##STR1## in the formula, R', EO, and PO denote an alkyl group, ethylene oxide group, and a propylene oxide group, respectively, and m and n show an added mole number. ##STR2## in the formula, X denotes a hydrogen atom or a hydroxyl group, R.sup.1 and R.sup.2 denote a hydrogen atom or a methyl group, AO denotes an alkylene oxide group, and p shows an added mole number.